1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same and, more particularly, to a current path material for electrically connecting an electrode formed in a semiconductor device, e.g., the source electrode of a semiconductor element and the connecting portion of a lead frame, and a method of connecting the source electrode and the connecting portion of the lead frame by using the current path material.
2. Description of the Related Art
Many types of semiconductor devices have recently been shipped as products. One of these semiconductor devices is a semiconductor device called a MOSFET 101 sealed in an SOP-8 package, as shown in FIG. 1. A conventional semiconductor device and a method of manufacturing the same will be explained by exemplifying the MOSFET 101 sealed in an SOP-8 package.
As shown in FIG. 1, the MOSFET 101 is sealed by a mold resin 102 made of an epoxy resin or the like. As the name of the SOP-8 package implies, the MOSFET 101 has eight lead frame terminals 103. The lead frame terminals 103 are exposed outside the mold resin 102 such that the lead frame terminals 103 are grouped into four to face each other on both sides of the mold resin 102.
The main part of the internal structure of the MOSFET 101 is constituted as shown in FIGS. 2A and 2B. FIG. 2A is a sectional view showing the MOSFET 101 taken along the line A—A in FIG. 1. FIG. 2B is a sectional view showing the MOSFET 101 taken along the line B—B in FIG. 1. Of the eight lead frame terminals 103, the four lead frame terminals 103 on one side are combined into one within the mold resin 102, as shown in FIG. 2A. As shown in FIGS. 2A and 2B, the four lead frame terminals combined into one are arranged within the mold resin 102 so as to be electrically connected from a side (upper side in FIG. 2A or left side in FIG. 2B) opposite to a source electrode 104s and gate electrode 104g of a semiconductor element 104.
As shown in FIGS. 2A and 2B, the remaining four lead frame terminals 103 are arranged within the mold resin 102 so as not to be directly connected to the semiconductor element 104 including the source electrode 104s and gate electrode 104g and the four lead frame terminals 103 combined into one. Of the remaining four lead frame terminals 103, three lead frame terminals 103 on the source side are combined into one, as shown in FIG. 2A. The remaining one lead frame terminal 103 on the gate side is electrically separated from the three source side lead frame terminals 103 combined into one.
In the MOSFET 101 having this internal structure, the source electrode 104s of the semiconductor element 104 and the three source side lead frame terminals 103 combined into one are electrically connected by a plurality of bonding wires 105 made of a metal such as aluminum (Al) or gold (Au). Similarly, the gate electrode 104g of the semiconductor element 104 and the remaining one gate side lead frame terminal 103 are electrically connected by one bonding wire 106.
The recent MOSFET 101 is being improved in speed and performance, while being decreased in power consumption and operation voltage. In other words, the recent MOSFET 101 is being so designed as to exhibit higher performance at lower voltage. To meet these contradictory demands, the recent MOSFET 101 tends to be set to a low internal resistance (ON resistance) in the entire device including circuits along with micropatterning of the circuit.
If the internal resistance of the MOSFET 101 is decreased to follow this trend, the influence of the resistances of the bonding wires 105 and 106 on the internal resistance of the whole MOSFET 101 including the semiconductor element 104 cannot be ignored. To decrease the internal resistance of the MOSFET 101, the resistances of the bonding wires 105 and 106 must be decreased.
To decrease the resistances of the bonding wires 105 and 106, for example, the metal material of the bonding wires 105 and 106 is changed to another metal lower in resistance than Al or Au. This method restricts the kind of usable metal, and cannot greatly reduce the resistances of the bonding wires 105 and 106.
Merely changing the metal material of the bonding wires 105 and 106 cannot improve the performance of the MOSFET 101. It is very difficult to further improve a power MOSFET by reducing the internal resistance.
As another method of decreasing the resistances of the bonding wires 105 and 106, for example, the sectional areas of the bonding wires 105 and 106 may be increased. This method suffers various technical difficulties: spatial restriction considering the diameters of the bonding wires 105 and 106 and the numbers of bonding wires 105 and 106, the possibility of electrical short-circuits between the bonding wires 105 and 106, and poor bonding strength when pluralities of bonding wires 105 and 106 are bonded to the small-area source electrode 104s, gate electrode 104g, and lead frame connecting portions.
To solve these technical difficulties and decrease the resistance of the MOSFET 101, a MOSFET 111 has been developed. In the MOSFET 111, as shown in FIGS. 3A and 3B, the source electrode 104s through which a larger current (main current) flows than that through the gate electrode 104g, and the three source side lead frame terminals 103 combined into one are electrically connected using a current path material 107 (to be referred to as a strap 107 hereinafter) made of a flat plate-like (band-like) metal instead of a plurality of bonding wires 105.
In the MOSFET 111, the source electrode 104s and the three source side lead frame terminals 103 combined into one are connected by the flat plate-like strap 107. The sectional area of the current path between the source electrode 104s and the source side lead frame terminals 103 is larger than that of the MOSFET 101 in which the source electrode 104s and the lead frame terminals 103 are connected by a plurality of bonding wires 105. That is, in the MOSFET 111, the resistance between the source electrode 104s and the source side lead frame terminals 103 is reduced to reduce the resistance of the entire device.
Similar to the above-described bonding wires 105 and 106, the strap 107 is connected to the source electrode 104s and source side lead frame terminals 103 by conductive connecting materials such as cured conductive materials or solder. The MOSFET 111 having this structure is disclosed in, e.g., Jpn. Pat. Appln. KOKAI Publication No. 2000-114445.
In general, bonding materials such as cured conductive materials or solder used inside a semiconductor device readily generate failures with respect to temperature changes. To evaluate this failure mode, the MOSFET 111 undergoes a temperature cycle test a plural number of times in an environment where the temperature steeply changes. In this state, brittleness, cracking, and the like appear inside the cured conductive material or solder and near the interfaces between the source electrode 104s, lead frame terminals 103, and strap 107 and the cured conductive material or solder. The endurance of the MOSFET 111 having the strap 107 connected by the cured conductive material, solder, or the like upon temperature changes can be evaluated.
The strap 107 which is formed into a flat plate and connected to the source electrode 104s by the cured conductive material, solder, or the like exhibits an unstable electrical connection state at microscopic level. More specifically, a chip edge touch where the strap 107 touches the peripheral portion of the semiconductor element (semiconductor chip) 104 readily occurs at a portion Z in FIG. 3B, i.e., outside the source electrode 104s. As a result, an electrical short-circuit easily occurs between the strap 107 and the peripheral portion of the semiconductor element 104.
In this way, the electrical performance of the MOSFET 111 with this internal structure is unstable. More specifically, initial short-circuit failures occurred in 18.5% of the total number of MOSFETs 111 manufactured as samples.
It is an object of the present invention to provide a highly endurable semiconductor device which can operate at low power consumption and exhibits stable electrical performance, and a method of manufacturing the same.